1. Field of the Invention
The present invention relates in general to an optical device, in particular, an optical voltage sensor or optical modulator utilizing a light beam in particular, and more specifically to an optical unit or component used for such a voltage sensor or modulator which utilizes an electrooptical effect.
2. Discussion of the Prior Art
An optical device utilizing an electrooptical effect is known. The device includes an optical unit which uses, as an electrooptical element, a Pockel's element such as a single crystal of LiNbO.sub.3 (lithium niobate) or Bi.sub.12 SiO.sub.20. The electrooptical or Pockel's element is provided with a pair of electrodes. In operation, a voltage is applied between the electrodes to produce an electric field so that the electrooptical element is placed in the electric field. On the other hand, the optical unit receives a light beam transmitted through a suitable optical path such as an optical fiber. The incident light beam is linearly polarized by a polarizer, and the phase of the linearly polarized light beam incident upon the electrooptical element is modulated, depending upon the intensity of the electric field in which the element is placed. A change in the phase of the light beam is converted by an analyzer into a change in the intensity of the light beam emitted from the analyzer. The thus changed intensity of the light beam from the analyzer is sensed by a light detector such as a photodiode.
As described above, the electrooptical element used in the optical unit utilizing an electrooptical element is exposed to an electric field produced by a voltage applied between the electrodes. However, an electrooptical effect achieved by the electrooptical element is influenced by an external electric field other than the electric field produced by the voltage applied between the electrodes. Accordingly, the quantity or intensity of the light beam to be detected by the optical unit is undesirably varied due to the external electric field.